If I understand this correctly
- You've been together 5 years
- You live at home with one 14yo
- Your partner lives in a different house and his 14yo spends 30% of their time with your partner, and 70% with their other parent
- Your 14yo doesn't want you and your partner to move in together because they like going to the loo with the door open and walking around in their pants
- The two 14yos would not live together. You and your partner would live with your 14yo, his 14yo would come and stay, presumably 30% of the time.
I'd want to understand if there was anything deeper sitting behind your 14yos reasons than just general 'I don't like change' because they're rubbish reasons.
And I'd want to understand whether your partner's 14yo is worried they'd lose one on one time with their dad if he was part of another family.
I'd need to know whether the two 14yos broadly get on or not.
And whether they'd have separate rooms.
But assuming they each like you and your partner, get on ok and would have separate rooms, I'd move in together now. They're only seeing what they might lose, and as long as you make it very clear that they wouldn't lose anything worth having (close the door when you're piddling, you piglet, but we'll still have special us time - let's schedule it), then they could gain a lot from this relationship. I'm a firm believer in love growing, not dividing, and the power of good step-parents to add to a child's life, because that's what I have experienced myself growing up.
If you have ANY doubts about your partner, at all, or if anyone doesn't get on with anyone else, you have to wait until they're adults I'm afraid.