I have Quote Line Entity in Solution-1. I have customized "Add New Record" (+) button in Solution-1. Similarly I have customized "Add New Record" (+) ribbon button in Solution-2. Both of them are managed solutions.
If I Import both the solutions one on top of the other, Will one over write the ribbon customizations of previously installed solution ?
Yes, it would, remember customizations essentially all boil down to XML. CRM stores customizations as XML, so importing solution on top of another which target similar attributes, elements end up overwriting the XML (customization).