Maxperson
Morkus from Orkus
It seems to me that spells require components like somatic or verbal. If you are using your math or free hand to cast a different spell then it seems reasonable to rule you can’t use that same mouth or free hand to cast counterspell.
So IMO no house rule is needed. Just an abdjuction of the rules.
Components are pulled out when you cast the spell. They are also often not cast at the same time. I could see a concentration check being needed in order to counter a counter headed at one of your spells, though. You're sort of interrupting yourself to counter the counter.