According to John Keith :
I'm helping an elderly couple with some troublesome windows in their
home. The windows are sliders (one slider on each end) separated by a
48" fixed window. The windows are old and the latches don't work well
and the seals are poor, therefore they leak air badly. I have done
some weatherstripping which is an improvemetn but what I am looking
for now is a device that can be inserted between the two sliders and
exert some pressure on each side to force them to close tightly
against the far edge. I'd like something that has an over-center type
action or a screw adjustment to apply the pressure since neither
occupant can handle much force. Before I go off and try to fabricate
something for them can anyone suggest a commerical product that could
accomplish what I am trying to do? If further explanantion is needed
I'll gladly respond with more details

It sounds as if you want to rejuvenate the seals (such as they were)
and provide a mechanical assist for closing the windows.

Depending on the geometry, simple slider locks might do the trick. The
ones I'm thinking of are small devices that screw onto the bottom window
frame, that have an "L" shaped nylon handle. When you push the handle,
it pushes a wing of the L against the glass (or sash) and jams the
window hard against one side of the track. Which will help better
seal much of the perimeter, especially if you put a second one on the
top frame.

Something like this device: http://www.allglassparts.com/product/784095899

http://www.allglassparts.com will probably have something you can
use to help seal the windows better, as well as something that can
give a mechanical assist to close them tight. You might even be
able to use something like old fashioned vertical sliding window locks
for tightening the windows.

Sashless sliders often have felt strips glued to the glass. On
aluminum sliders, felt strips locked in grooves.

http://www.allglassparts.com looks very much like a "if we don't
got it, it don't exist" type of place. Spend some time surfing
it, I'm sure you'll find stuff that will do the job.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.