New Player, I summoned Karla and Legault what can I do with them?

You can absolutely put them on the same team!

For the most part, they've already got all the tools they need to thrive, especially for a new player without resources to spend yet. But considering they both lack defensive stats, you'll want to put them on a team with a character that excels there.

Someone defensive with reposition to get the more fragil unitsnout of danger. Brave Ike is a good choice, if you have him.

If you're brand new, you could pair them with your Sharena, since her defensive stats and support abilities are good. You might have to start leveling her up and promoting her, though.

Congratulations on pulling both in 40 orbs, that's fantastic! I recommend Fjorm to go with those two because she's low-investment at the beginning and takes magic damage very well and works pretty well against non-green physical damage in PvE. Once you unlock the sacred seal forge, give her the QR seal (even level 1 will augment her power considerably). Play her a lot on Fridays and Saturdays, because she has a lot of high-SP skills (ice mirror alone is 500, not counting sacred cowl) and you'll want the 2xSP bonus.

You might consider using Anna because she's quite fast but doesn't hit that hard, meaning that Legault's def/res penalty on opponents will help her a lot.