Viagra (sildenafil) at 25mg doesn't work for me. Can I swallow two 25mg tablets in one go to make it essentially a 50mg dosage? Or is that not a good idea at all?

Viagra (sildenafil) 50mg. is not a particularly worrisome high dose. A more important question is why you need it in the first place, even 25mg. Please confer with your doctor to make sure all organic causes of ED have been excluded.