Double Team (and indeed all evasion moves) are kinda frowned upon, and are banned in Smogon tiers (except Ubers).
I would go with something more useful to be honest. Double Team only increases the chance of Jolteon surviving a hit, it doesn't guarantee it, and Jolteon already has low defences, so you're probably just going to waste a move slot with Double Team or Agility.
Jolteon @ Choice Specs
Ability: Volt Absorb
EVs: 252 Sp. Atk, 252 Speed, 4 HP
Nature: Timid (is best)
- Volt Switch
- HP Ice/ Shadow Ball (depending on what its HP is)
- Signal Beam
I mean, you could use your Jolteon to baton pass Agility to a sweeper, but like I say, it's quite frail already, so you'll want to use it to sweep as fast as possible.