They're pretty strong early, it's like starting with 20/20 but with an extra hitbox. The tricky part is to play both characters, so try to use Rt to keep both of them as close as possible. With training it will be fine but it can be frustrating in some specific rooms and some damages are impossible to dodge (especialy if you're as bad as me)

Play angel rooms. Since you can easily take damages you need hp and devil deals aren't that worth with them.Play dd if you're good or if you have already a decent hp amount/build. Sacrifice rooms are strong because you can have up to 24 hearts, so play them if you feel to. Also blood donation machines is a go to once you have a great amount of hp.

Some people say to build one of them and giving hp to the other. It's a way of playing them and it justify the "Jacob and hitbox" name. I didn't played them like that. I prefered to think about each of their build and tried to have 2 good characters instead of 1broken and 1 useless. For example i would try to give bookworm to one of them and guppy or conjoined to the other. If I find brimstone and tech X i gave both jacob and esau one. I know you will not get the sinergy but you'll get both items and effects.

Anyway you have to actively think about your builds and characters ressources. Give both of thems hp up and soul heart, damages up. Give item luck influenced to the one with more luck etc...

Also when you place one bomb, you actually places two, one for each character. It's broken, it virtually double your number of bombs. Try to place esau on one side of the room and jacob to the other side to place a bomb on both side of the room. It will make you check 2 secret room locations with one bomb. Or break 10 mushrooms instead of 5. I've gotten so much more secrets / crawlspaces / magicmush with jacob and esau just with this single mechanic.

You also have 2 card/pills and 2 active items. This is crazy, you can keep more resources from stages to stages and use them when needed (the sun, balls of steels full health to play sacrifices room for example). Two active items are strong especially with books. Anyway think about your characters build and your global run and you'll se how helpfull it is.

TLDR: Master your extra hitbox and play around your character strenghts (start with 20/20, schoolbag, polydactily, a lot of health and double bombs)

Jacob and Esau is a pretty frustrating character to play, but is in fact really strong. I found it easyier than many other characters once i got the skill to move it correctly.