Any advice on sealing together 2 large glass panes 2 make one big sheet. My glazier said it wasn't possible without (expensive) drilling and making a frame. Like a shop front - I looked at a shop fronts and they appear to have some sort of solid mastic.

I aint no glazer but I think I know what your getting at, I think you'l find shop fronts are very thick glass making them very rigid & are sealed after being fitted into the frames, similar to aquarium tanks which are held together using just sealant alone.

what are you planning with the two sheets and roughly what size are dg or single and are they tough or float glass.you can buy soffit joiner which has two groves in and is only 10mm wide so is that what your looking for.you can put the units in and have a clear mastic bead down the centre but you need to be good with a silicone gun and the main problem with mastic on glass is it peels easy.you need to sit them in a secure frame also.