Rydyn ni'n caru'r ddau
Here's a breakdown:
* Rydyn ni - We (plural, informal)
* 'n caru - love
* 'r ddau - both
You could also say:
* Rydyn ni'n caru'r ddau ohonyn nhw - We love both of them
* Rydyn ni'n caru nhw'n ddau - We love them both
The choice depends on the context and who you're referring to.