I am in 2nd year History at Glasgow and have taken Archaeology as an elective for first and second year. At the end of this year I can add it as a joint if I wish, I am not sure hot works in all universities but Scottish universities you select 3 subjects so 2 more if a single honours and 1 more if a joint and you can add/ swap and change. Maybe worth researching or emailing the universities you are interested in and seeing the possibility of dropping one if you do not enjoy the joint.
If you enjoy both it is definitely worth it