As mentioned, I wouldn't silicone either as you need a few days for it to cure and for the fumes to go away.
Cardboard would possibly work for a couple days, but you need to cut it to fit perfectly.
I built a Styrofoam background for my 90gal for under $50 including high quality Styrofoam, grout, glue, paint, and sealant. You could easily just cut styrofoam for a tight fit as a divider, which wouldn't cost you much
If you divide your 40gal breeder, how are you going to manage with maintaining temps? Do you have an extra UTH?
If so, I would then recommend you just go to Walmart and buy a tupperware/plastic tub with click-lock lid. Probably twenty bucks, you just need to regulate your heat.
Regardless of what route you choose, you have two snakes you must ensure are comfortable. That means they both need water, proper and regulated temps, and warm/cool hides. I don't think you can avoid spending at least $50 to ensure both snakes will remain healthy and happy, even as a temporary means.