This is a really difficult situation. As you are aware, 'the system' will take his income into account, even though he's not your DC's father, so has no responsibility for them. Plus, if his 'really decent wage' is above £60k, then not only do you lose your UC, but you also lose child benefit. Then if your DC go to university, his income will be taken into account when assessing what loans they're entitled to, with the household being expected to bridge the gap, even though you personally probably don't have the means to save up a significant amount or contribute if this is the path they take.
Although, unless your DC have additional needs, won't your UC entitlement be based on you working more or less FT anyway?
But then he could argue that you should pay more than 50/50 because you're also housing and feeding two teens, although it's likely he has more expensive tastes than you so it could balance out a bit.
It could be that either you don't do 50/50, especially if you end up getting married, where he's committing to 'sharing all he has with you' and taking you on as a package, which includes DC.
Or you do 50/50 but you need to choose housing, bills, food etc based on your means not his, so you're not skinting yourself paying for a large property in an expensive area. Or you wait until the DC are independent adults - what's your plan for when you're not entitled to UC anyway? Even if you work FT, it's going to be hard on NMW if you don't get help with rent etc.
You don't mention their father. Does he contribute?