Parrotlets are a different species of parrot than lovebirds. But, either way, you should not put them in the same cage. You should always be careful when your birds even have contact with eachother, especially when they are different species. 5 weeks is very young. Are they still being handfed? When they are that young you have to be especially careful not to stress them, as they are very susceptible to illness. And make sure you quarantine birds from eachother when they are new to eachother.