From what I have found, each party member will gain EVs even if they did not participate in battle,
If you don't want to be constantly updating your party, this leads two effective strategies to be:
- Farming money to buy Vitamins.
Vitamins are items that increase EVs all the way up to the max amount.
But they do cost quite a bit, so you will need to farm up some money in game to buy them all.
- Sending Pokemon on seminars from the Job Board in Pokemon Centers.
This allows you to train up to 10 Pokemon per stat at once, but it takes real world time to do.
More detailed information can be found in my source, but I have also now tried all of this myself to confirm.
So the most effective way mostly depends on your play-style:
With lots of play time you can just farm up money with an Amulet Coin (found behind the Motostoke outskirts sign) and buy all the vitamins,
you can also play the Battle Tower and buy vitamins with BP.
Vitamins purchased can instantly fully EV train a Pokemon, you would need 25 for your two stats, bringing them to 250 each, then either a 26th vitamin or one hour with the below method to hit the 252 max.
Or you can log in daily and send up to 10 Pokemon off on a seminar, then return the next day and do it again. From my experience sending a Pokemon off for a whole day rewards 96 EVs. There are however mulitple time options to suit your playstyle, all awarding 4 EVs per hour:
Timeframe Text Actual Time EVs Obtained With PowerItem
Whole Day 24 hours 96 288
Half Day 12 hours 48 144
Very Long 8 hours 32 96
Long 4 hours 16 48
Short 3 hours 12 36
Very Short 2 hours 8 24
Just a little 1 hours 4 12
As noted on Serebii, having your Pokemon hold one of the Power Items will also increase the EVs obtained from the jobs.
Buying these power items is a great investment, each adds +8 EVs per hour to the Pokemon holding it, effectively tripling the amount of EVs gained per hour!
It will not take much game time, but this method could be more real world time as it will take several days to fully train a Pokemon, unless you have the power items, then it can take just over two days (one full 24 hours for each of the two stats, then you use another hour for the remaining third stat for 4 EVs).