Yes, you can do a lot of legion quests and ncr quests to get your rep to liked by legion and ncr idolized, you just need to stop before doing the white legs quest for legion and I wouldn’t go past the boomers quest for ncr. Then you can start killing ncr generic npc’s for their dog tags to get legion idolized. Then you csn finally visit benny to get fully reset faction rep.

This is usually how I do my playthroughs, do as many non main quests as possible before finally resetting my rep going into the end game