Keep render layer A while re-rendering only render layer B

Hi everyone!

I’m rendering a face portrait and decided to split the render into two layers: one for the head and one for the facial hair fuzz. I set the two layers on in the render layer tab and rendered.

I accidentally forgot to set “Transparent” on, so the hair fuzz has no alpha. I just wanted to re-render only the facial hair layer (since the head is ok without alpha for me) , but once it finished, the head layer was no more in the compositor. Is there a way to keep it if I have to redo only the other layer?

Sorry if the question may seem stupid, but I’m pretty new to Blender and I couldn’t find a solution on the net!

Thanks in advance!