I probably wouldn't use 2 spring lip seals in tandem or it may starve the outer one, as you point out, making all that extra work a moot point. If you've ever had to deal with lip seal grooves, whether on axles, or crank balancers or torque converters, etc., you know that eventually, lip seals will wear into the metal basically rendering the nominal seal used in that application useless. If you're spring lip seal is leaking and it's a problem getting it to seal, there's a reason other than not enough seals.
Spring-aided seals can be part of the issue with eating the seal surface over time, but they're also the beauty part of the equation. Many variables can come into play- speed, pressure, temperature, etc., so I'm going to be nebulous on this, but there are lip seals that aren't spring-aided, and they do well in very low pressure applications, like being vented on both sides, and in situations where maximum sealing isn't a huge concern. O-rings, E-rings, and labyrinth style seals and the like can do the job where bleedout may not be a huge concern and are more for static sealing conditions. But one of the cool things about the spring-aided seals is that they help seal even with conditions of in-tolerance run-out of the shaft so the slight pressure on the lip of the seal that the spring gives helps you leak less over time. The durometer of the materials in a spring seal are usually firmer than non-spring seals, so the spring helps keep good pressure on the seal to help prevent lip seal set issues and still seal even if there's slight reciprocating movement of the shaft. This helps the seal do its job in harsher environments for longer period of time.
Packing the void between 2 seals wouldn't be advisable, IMO, because there's a risk of the grease could just hold heat, and the grease closest around the shaft would start to liquify and whip up and perhaps cause swelling of the volume and raising internal pressure between the seals, and excess grease would go somewhere. Which probably wouldn't benefit you in any way.
There's likely a reason the engineers designed the gearcase to use the seals the way they did. If you use the wrong material or size, etc., it can cause you issues as well. Ensure the seal surface is flat and smooth with no pits and/or burrs. You don't want any rough surfaces to eat the seal. If the shaft is too far bent, then it won't matter how many seals you jam in there.
I would just ask the question to myself...how many single lip seals does my car use and need? Why am I only needing one seal on the harmonic balancer or torque converter, drive shaft yokes, etc.?
All this probably doesn't help you much on deciding, but it's just something I would consider.