It is not possible to have Double-Up & Gemini at level 50. Therefore, I assume you will either be level 61 (Ultimate Vault Hunter Pack) or level 72 (Ultimate Vault Hunter Pack 1 & 2). Since this is a new question, I'm going to guess you have the Game of the Year Edition which only goes to 61.
At 61: Double-up & Gemini is a fun combo. Your turrets do most of your damage, while Axton brings little else when the turrets are on cooldown.
Longbow is exceptionally useful for placement, plus the increase life gauge. Instead of just "lobbing" the turret(s) somewhere close, you can accurately put them where you want. This is very helpful in various situations, as you can position strategically so not to take as much damage (from a boss, for example).
In UVHM, slag becomes exceptionally important and almost required. Because of this, Double-Up is far more useful. If you use Double-Up as a partner (as in you focus fire on whatever the turret is currently attacking), you don't have to slag enemies and can put out a lot of damage. If you choose not to get Double-Up, you will have constantly slag enemies yourself (swapping weapons), while the turret(s) is/are over there just doing damage.
There isn't a 'better' way. It is more dependent on playstyle and how you want to use the turret(s).
In my opinion, I can't justify wasting 25 points into the Survival tree to get Gemini. Of the given options, I would choose (at UVHM) Double-Up + Longbow over Double-Up / Gemini, and over Gemini / Longbow.