It is highly probable that the steam will go quickly to the new radiator and the condensate will have to pass through the steam flow in opposition to the steam. This would increase the probability of banging radiation. It is best not to burden the first radiator with the added condensate from a new one on the opposite side of the radiator from the steam source. The runout for one radiator is usually sized to the steam need of the individual radiator, not two. Also, the new radiator would have to be higher by one inch per 10 feet to help the condensate back to the first radiator, which in turn would have to pass the condensate of the new radiator in addition to its own. In short, take the new radiator off the main.