@dabbadabba,
OP:
1. The baseboard and door installation are underway and nearing completion.
or
2. The baseboard and door installation is underway and nearing completion.
Phil:
#2 is correct.
#1 would be correct if there was more than one installation:
The baseboard and door installations are underway and nearing completion.
Dale:
Db, I can only incidentally add that if the baseboard and door were somehow very closely connected so that you might hafta do both at the same time, that in #2, 'is' might be okay