I have been spending more than my maximum cashback limit on UOB One recently. Should I get a a miles card as secondary card?

I am going to say a unpopular opinion.

No.

If your game plan evolves around cashback cards, getting a miles card is just going to complicate things. Krisflyer miles expire in 3 years so if you don't monitor close enough, it will be gone. And besides, how much miles are you getting each time? Just to give a basic point of view, to redeem a one-way biz class ticket to Malaysia/Brunei, it is from 19,000. A round trip will be 38,000. Bangkok is 21,500 one way, making it 43,000 round trip.

I just want to say, unless you have a gameplan in mind, don't just accumulate miles as a side thing just because it seems to make premium cabins more accessible. Either you don't accumulate enough within 3 years or the miles you accumulate are just too little to be of any use.

Having said that, if you don't mind spending the extra effort to monitor your miles balance, get the DBS Altitude card and Citi Premiermiles card. The points earn don't expire so you can take your time to accumulate enough before converting them to Krisflyer points.

Cheers

Do you expect to be consistently spending more than the max cashback limit?
If it's for travelling for work like you said (and I guess business expenses are reimbursed), then no harm looking for another card to accumulate more points!