Yes it's nice to have 2 systems. one studio and one for mobile. As I just said I swapped duty and now use my Tascam for the studio and the Scarlett for mobile.   Reason is I am at this point not recording any bands. I've set up my laptop and once I put my parts down it will go to my partners house so he can record his parts, Banjo, vocals and a 12 string. He's retired so has lot's of time on his hands and will be good distraction for him learning how to record with a DAW. 
Curt is an amazing banjo player but we are both getting on and we need to document these old songs before we fade away :)  We have over 60 songs we will record. 
 
Are you going to look for a second interface? Possibly the Tascam 4x4?  I like the looks of it. 
http://tascam.com/product/us-4x4/specifications/
 
As you can see all 4 channels can be used for either, mike line or instrument. And it has the mix control on the front panel. 2x headphone out as well. 
 

That is one choice but I'm "gun shy" about Tascam.  I tried an 8 channel unit and the headphone out had low level hum in it (it was in the Tascam unit as I didn't get any headphone hum with any other unit). But for a backup unit that may never be used it may be OK.
 
Not sure what I'm going to get, but it will have at least 4 mic/instrument preamps.
 
I still need 8 channels for recording.  I did an "on site" recording yesterday of our Cowboy Church band (I'm one of them) plus two singers (the Pastor and another singer).  I had two channels for the drums, one for the Bass, one for the lead guitar, one for my Pedal Steel Guitar, one for the Rhythm Guitar and two for singers.   I did a live recording of our regular band, at a show, about 6 months ago; it was two for drums, one for bass, one for lead, one for my steel, one for rhythm guitar and two mic's for singer and backup singer.   For the drums, I have one kick drum mic and I have a Ribbon that use as an "overhead" for the rest of the drums - works good for "country".
 
I probably got you and the friend beat.  I was 80 last December (and still going like I did when I was 60)......
 
Jack 

Ha Ha, Wow I didn't anybody here was older than me..( 65)  I think you get the prize for that Jack ! 
But ya, we've been playing music together for over 30 years now. 
It was like this Circus act I saw once, The young people were all called the AMAZING such an such , the old guy was called the LEGENDARY. 
 
My Tascam us1641 headphone output is wimpy but quiet. 

Well, the 6i6 is gone. Sold to a guy in California.  Now to look for something else.
 

Picked up a new Tascam 4X4.  Good enough for a "backup" unit that will mostly collect dust.