A lot of times, when putting on combo orders, Interactive Brokers, will fill it using LMT + MKT algorithm.
Both legs are put in limit. Eventually one side fills, then the other immediately switches to market.
There could be lots of variations on the execution algorithm. The issue really has to do with momentum.
If you want to go long A and short B, and A and B both moving up, then you probably want to get the long side in first. Long A however you can, limit or market or whatever. Get that in first, and wait for it to fill. Then put the B order in as market.
Then sort of the opposite approach if both are moving down. You would want to get B in first, in that case.
Now, in a mixture, one moving up and one moving down, it is more tricky, but I am sure a thoughtful analysis would yield some appropriate strategy. Maybe, whichever one is moving faster, get that one on first, for example.
You want to avoid bad fills, first and foremost. If you can get "fair" fills (e.g. close to the mid-point) you are probably doing pretty good.