On the old brass radiators often, there was a tin U shaped channel soldered between the tanks for stiffness. If that's what you're talking about I guess 2 holes wouldn't affect the integrity, but you better not be pushing hard when you drill the holes.
I saw a guy doing this with a brand-new radiator on a stock car and drilled right into the core tubes. The guy that owned the race car was less than pleased. Luckily the guy that owned a radiator shop was a friend of mine, and soldered it shut for N/C.
