the stress of twins milking a cow is dependant on the cow.
Out of 370 ish Limo full and pure bred calvings here I have had a single set of twins, one left on mom, and the other placed with a second twin from a comercial cow on an old milk cow. The milk cow had been milking in a local dairy herd for like 18 months, then raised three calves in a year, and supplimented a few more.
Comercial cow twins here I guess were around the 2% mark. Only two sets remained as twins the rest pulled off. Calving ease wise I have not helped in most of the twins. Generally assistance has been below 5%, even this spring when I had 120 heifers.