The thing is they are most often relatively near in power (at least portrayed that way), but often there are counterexamples where they are quite far away from each other.

• Film 4 Endgame: There were statistics by the watchers how many quickenings everyone had. They even said, that both McCllouds by themselves had no chance as the other guy had double their quickenings.
• The Series: McCloud battled a very old immortal just before going to Paris. IT was said that he had almost no chance due to the other one being extremely powerful and old. And even if he had, the chance was there that he would become evil (as the other was such a strong evil immortal).

The thing is they are most often relatively near in power (at least portrayed that way), but often there are counterexamples where they are quite far away from each other.

• Film 4 Endgame: There were statistics by the watchers how many Quickenings everyone had. They even said, that both McCllouds by themselves had no chance as the other guy had double their quickenings.
• The Series: McLeod battled a very old Immortal just before going to Paris. It was said that he had almost no chance due to the other one being extremely powerful and old. And even if he had, the chance was there that he would become evil (as the other was such a strong evil Immortal).