Can I stack the miles earned on my miles credit card with the miles earned on the KrisPay app?

Answer is yes. I am going to break up your questions into a few parts.

For a merchant that offers miles via krispay, at a rate of 2mpd, you earn 3.2 mpd if you pay via cards that earns 1.2mpd, or 3.5mpd, if you pay via BOC Elite Miles. Basically, it is 2mpd + the base rate for whichever card you are using. The only difference is that 2mpd goes to your Krispay app, which you should transfer over to your Krisflyer account within 7 days from the transaction. Miles, once transferred over to Krisflyer, has a 3 years validity.

The 1.2, or 1.5, or whatever the base rate is, goes to your credit card points/miles, which is then subjected to whatever expiry date the bank sets.

Confusing? Let me use an example. The merchant, We Need a Hero, offers 2mpd when you use Krispay app to earn miles. Let say you spend $50 with them. So you pay via DBS Altitude Card and earn 60 miles (1.2X$50). These miles are kept in your DBS credit card account, which never expires. You then open up the Krispay app, enter the amount you paid, generate a QR code for the merchant to scan. After scanning, you earn 100 (2X$50) Krispay miles. Within 7 days, transfer these 100 Krispay miles at a 1:1 ratio to your Krisflyer account. The 100 miles that are transferred over will then be valid for 3 years.

So in total, you earn 160 miles, of which 60 is in your DBS account, which nevers expire, and 100 is in your Krisflyer account, which expires in 3 years. And the mpd is 3.2 (160/50)

Article for reference: https://milelion.com/2019/03/06/finally-krispay...
Hope this helps.

https://mainlymiles.com/2019/03/06/you-can-now-...

For your case, you won't earn 3.2 mpd. Credit cards have exclusion of payment to top up another payment account.

You need transfer krispay miles to krisflyer miles within 7 days. Then it will be 3 years expiry.