As Joe was suggesting, having two pianoteq instances is the way to go. But you don't have to be confined by the features of standard vs. pro to do the split, assuming your sequencer has the right features to do it.
For example, I use Reaper, and there you can use a simple "midi effect" called "JS: MIDI/midi_note_filter", to set the active range for a track.
You set different instances of this filter for different tracks/instruments.
Then you create a single track for you keyboard and route it to all these instruments.
This way you can split (and/or layer) not just two instances of pianoteq, but k-instance of anything - e.g. pianteq in the right half of the keyboard, and some other Bass vsti for the left half, layered with a cymbol, giving you a "piano jazz trio" setup.

Other sequences should have similar capabilities. Sorry for not laying down explicit directions, but I hope this would give you a couple of ideas.

Regards,
Eran

I just read this thread on 22OCT2011, even though it was authored 08SEPT2011.

There is a way to control two Pianoteq sounds on a single keyboard, even if the keyboard does not allow splitting itself into two midi channels.  Aside:  I use Pianoteq PRO, and am not sure whether this can be done with the Standard version.

If you wish to achieve the marimba at notes lower than C4, and have the vibraphone play all higher notes starting at C4, do the following:

1) Create two instances of Pianoteq and keep them both as recognizing Midi channel #1. 
2) Now, to get the vibraphone to play all notes starting at Middle C, simply edit the sound such that the volume is zero for all notes below C4.
3) To get the marimba to play all notes below Middle C, simply edit the marimba's volume such that its volume is zero for all notes above C4.

Happy split-keyboard playing!

Cheers,

Joe

Also, a different question about splitting in my previous post:
http://www.forum-pianoteq.com/viewtopic.php?id=2029

Thank you for your help!