MIDI Routing

Not exactly. SONAR does not have a built-in facility to merge or bus MIDI track outputs, but you can put the two parts in separate lanes of a single track and get the same effect. You won't have the independent control and editing of parts that you would have with two tracks, but for a piano part, you usually don't need that. The one major exception is if you want to edit notation in SONAR's Staff View after import. In that case, you need two tracks to get the two clefs displayed separately.
 
If you really need/want them to be on separate tracks for this or other reasons, there are third-party virtual MIDI cable solutions (some free) that can be used. There have been a bunch of thread about this.  I think this is a good one, although it appears the image hosting for the included screenshots expired:
 
http://forum.cakewalk.com/tm.aspx?m=1573767

I think that's what they are talking about, not much use for midi effects really. So I think you mean VST instruments. 
 
It's very easy to have any combination of tracks for midi. You can clone, copy, separate at any note value and create 2 or 4 or 6 tracks for each part and on and on. Because midi is just data it is easy to work with. This has been going on since 1980's and has not really changed or improved much. Any DAW will set up midi tracks the way you want.  Even the one your already using. 
 
I think what you want to do is import a piano part and then assign that one or 2  tracks to a number of different soft synths. 
No version of Cakewalk allows multi outputs. You have to have a midi track for each instrument. You can assign as many tracks as you want to one soft synth but not the other way around.  
So like your piano part, you import the 2 tracks and assign them both to True Pianos. 
Once you have your tracks you then need to insert and assign a soft synth for each track.
So all you need to do is clone or Copy that original track as many times as needed and change the outputs of each to the desired synths.
 
Cloning = All editing will effect all the tracks globally.
Copying = The tracks can be edited individually. 
 
If you use TTS-1 as a synth for GM sounds you would want to change each tracks channel. TTs-1 is a multi channel synth. You can create  16 instruments with one instance of TTS-1.  Synths like True Pianos are only one channel. But you can insert more than one instance if you like. 
First you have your piano part playing then you can layer up other sounds. 
You could take your left hand piano part and assign it to an Acoustic bass. 
Take the right hand part and assign it to a string section as example.  
 
You certainly will have more and better quality VST instruments to use if you upgrade to Professional from Home Studio. I believe your cost is only $99 so a heck of a deal really. 
 
There is very little difference in all versions of Cakewalk so if you have mastered Home Studio that's a big part of using a newer version. 
 
Spend as much time as you can watching the tutorials and reading the help files or pick up the Sonar Power book. You'll soon be a midi wizz.  

Not a plugin I've come across before, nor do I use Maple (I'm entirely 64bit and the developer's site says it's 32 bit only so I can't even test it).

Having said that, if Maple is providing you with functioning MIDI in and out ports, I'd suggest trying this....

As far as Sonar is concerned Maple should look no different to any other Windows MIDI port. Sonar won't echo incoming MIDI to its track output port unless input echo is enabled on the track. Which prevents hardware synths that don't have a "local off" function double-triggering, first from their own keyboard then again from the MIDI coming out of Sonar.

So if you've not already tried it, enabling "input echo" on the tracks receiving input from Maple might get things to work.

Failing that, I'd just clone the MIDI track into enough clones to give me the number of MIDI tracks required and drop the plugin into each of them. The disadvantage being that if the MIDI to all the tracks needs identical edits it's no longer possible to do by editing just the one track.

Cactus Music
Cloning = All editing will effect all the tracks globally.
Copying = The tracks can be edited individually. 

A special thanks for that. To be sure you have just saved much hair pulling and opened up a powerful and time-saving feature for me.