welcome to the forum. My OH didn`t use viagra every day, just `as needed` and we found that a longer time lag between taking it and using it worked well for us. Probably about an hour and a half time lag. Taking 2 pills didn`t shorten the time lag so 1 pill was as effective as 2. We were told that if it didn`t work there were other options but we didn`t need to investigate those so can`t be any help, sorry.
Oh, and my OH had previously had problems due to a partially blocked heart artery. A stent solved the problem. Apparently it can be the first symptom of a heart problem so you might want to get that ruled out.
Good luck and best wishes to you and your partner.