Can I combine the miles earned from different UOB credit cards for conversion?

Spending on UOB credit cards does not award you miles directly. Instead they are rewarded in the form UNI$ to be redeemed into miles.
Not only do Miles/Travel card earn miles. Reward cards such as UOB Preferred platinum visa (PPV) earn UNI$ which can then be converted into miles.

In fact not just UOB Cards work in this manner, DBS Cards work in the same way as well.

So to answer your question, Yes you can (with the exception of Cashback/Rebate cards)

It will be best for you to clarify this with UOB, I believe.
Personally I am not a miles card person, so I wouldn’t know.