Sampler that allows different samples for legato?

I am no expert at this but I think what you are talking about is more a feature of the sample set and the instrument definition than the sampler itself.  In Kontat, for example, a library or sample set can have legato samples that are triggered by overlapping notes or by the velocity of the note. If the samples don't have a legato articulation then the sampler could do a pitch bend or something to fake out the legato but it would not be a true legato.

There certainly must be someone on these forums that has more knowledge about this then me but I will give you what I know.
 
In summary - A sampler itself is not going to magically make some sample set have legato. The samples have to have been recorded with a legato articulation OR the software has to have a way to mimic the effect. If the bass samples have a legato articulation you should be able to set up the sampler to trigger the legato in various ways but you will have to setup the sampler appropriately to trigger the legato samples in the way you want.
 
Read below if you want to know how I got to my summary
 
I think what you are saying makes sense. In a literal sense a sampler is just something that can record snippets of sound, and in an expanded definition, give you tools to set things like loop points.  In the olden days of hardware samplers they often were more of a sample player then a sampler.  I had a Roland something or other and I never even created a new sample of my own. So basically it ended up being a sample player. It was already programmed with the samples and how to play them back. You could put your own samples in it and program loop points, samples to play at different velocities etc.
 
So that leads us to today where we can do all of this on our computers. There exists software that is called a "sampler" but most of them mean "sample playback and manipulation."  So as an example, Native Instruments Kontak is a sample player. I have never done it but you can import your own samples, program them to respond how you want and then play them back. In Kontak this is known as a library or a patch within a library.
 
So, as an example. if you have a bunch of bass samples in raw audio format and you want to play them in a sample player you would need to create a library or patch in a format that the sample player understands. This would tell the sample player what samples to play when a key is hit, what to do when mod wheel is moved etc.  One of these things could be to play a different sample, say a legato sample, if you hold the previous note down while playing the next one. Or it could be a key switch where if you hit say C1 then the next note you play will play back the legato sample.  However, this is all dependent on if the samples that are available to play actually contain a sample that was played with a legato articulation.  If there does not exist a legato sample then you would have to fake the legato sound by using some midi or software control like a pitch bend or volume envelope.
 
Sorry, for making this so wordy.
 
 

I use both Kontakt (Native Instruments) and Play (EastWest), and both allow complex scripting of samples in mapping them to keys and part of that is key switching, which is using midi notes outside of the instrument's playable range to change the articulation from one type to another, based on which key is pressed. So, if I am on a violin instrument, well below the playable range of the instrument,  number of the keys (usually the lowest ones) are scripted to be key switches, where each midi note of those key switch will be associated with telling Kontakt to use a different set of samples to play the violin differently. Various articulations will include for a violin things like: normal, legato, pizzicato, big vibrato, expressive vibrato (mod wheel controlled vibrato), etc. Some instruments, like violins, may have 20+ articulations, all controlled by these various key switches. Usually, the documentation for a sample library or instrument, will have a list of articulations in a chart, and I frequently print out such  chart when I am recording, so I can capture the right articulation for the recording.   

There are other ways to do it, e.g. using velocity as the determiner, or octave ranges (split keyboard).
 
However, keyswitching is the most flexible and is actually easily adaptable to live playing as long as there aren't a huge number of keyswitches. If you find it awkward to press a key during performance, or if your controller has a short keyboard such that it's impractical to assign to keys at the far end for switching, you can assign another control instead. If your controller has touch pads, for example, they can be used for switching instead of keys. Sustain pedal or mod wheel are other options.
 
Here's an example of using the mod wheel to crossfade between sample sets as an alternative to velocity- or key-switching. It's about brass, but the principle applies to any virtual instrument.

If you want one sample for the first note and different samples following that I would start with looking at how organ percussion is implemented in various samplers.
 
 
And the .sfz sample format has a "trigger" opcode in the sample definition that will do this assuming there's a sampler available that implements the first and legato triggers:
 

trigger
Sets the trigger which will be used for the sample to play. Values can be:

attack (default): Region will play on note-on.
 
release: Region will play on note-off. The velocity used to play the note-off sample is the velocity value of the corresponding (previous) note-on message.
 
first: Region will play on note-on, but if there's no other note going on (staccato, or first note in a legato phrase).
 
legato: Region will play on note-on, but only if there's a note going on (notes after first note in a legato phrase).