If you're just starting, and don't have many 5* or even 4* girls yet to fill your roster, then it might be beneficial to keep the dupes seperate for a while. Designate one as the "main dupe" and the other as secondary right away.
-Don't level the secondary dupe beyond lvl 40 (unevolved). It's really a waste of money to level her any further than that.
-Feed the secondary dupe to the primary dupe not too long after you evolved the primary dupe. Definitely before the primary dupe reached lvl 30 evolved.
-Don't feed ampys to the secondary, only manyus
Some of the leveling you did to the secondary will carry over to the primary when you merge the two, it's really very similar to merging two low quality rings resulting in a leveled low quality ring, which improves a high quality ring better than a non-leveled low quality ring would. But, even though you invested a dozen manyus in the secondary, it won't give the primary as much experience as an ampy would. So to still see some noticable increase in experience, you really want to merge when the primary is still relatively low leveled.
The nice thing about evolving the primary is, that it will restart at lvl 1, so it gets back down to a low level, and the merging will be more noticable in experience, while you still benefitted from having two good units for a short while.