What about the very popular and true doubles mantra... 'Get the first serve in'. I think people way under value the importance of getting the first serve in when playing doubles (even singles for that matter). Too often I see people trying to win the point with a big first serve that they don't even get in 50% of the time. Returning is one of my strengths so I start licking my chops because I know I'm going to see a lot of second serves and many people can have a huge first serve and than a duck of a second serve. In doubles especially... it's important to get the a good percentage of first serves in. The weaker your serve... the more important it is to get the first serve in. From the returners perspective there is always more doubt on the first serve of what is coming than on the second. With the first serve the server has every option available... they can hit hard, soft, spin, go for corners, body, etc. Second serve... most people are going to go with what they have confidence in and feels safe... and that usually means just two or three options instead of triple that even if they're decent players... huge advantage to the returner.
With a man at the net in doubles, getting the first serve in means more opportunities for the net man to poach and help out the server. It's much harder to poach on the second serve since the returner is going to see a very hittable ball. I have solid first and second serve, I can get an occasional ace and several unreturned serves, but I'm not dominant with my serve. I focus a lot on placement and variety and my end goal is to set up my net guy for easy put away... which I'm pretty good at doing. I always tell my double partners if they are struggling on server to 'hit up the middle' because it takes the angles away and makes it easier for me to poach. I also will tell them to 'just get it in and let them hit it' because it's not the servers job to win the point... really it's just to get it started/setup.
That said there is the occasional/rare player that benefits from hitting two first serves. A guy I play with hits 'first serves' on the both serves all the time. He's tall, hits flat, hits big, has a simple motion and can place the ball pretty well. If you're capable of getting close to 70% or better of your 'first' serve in... why not hit a second time if it's an effective weapon. He hits a lot of aces and unreturnable serves so he doesn't get broken a lot and the occasional double faults he sees... he gets back all the time with an ace or unreturned serve. It's a valid strategy if you serve is a weapon and you have the confidence to shake off doubles. I alway joke with him that he hits is his second serve as the first serve and first serve as the second serve.