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You can count yourself lucky if it's 10 years old. I have never had one come back that long. :)

When you trim it are you cutting it nearly to the ground? They bloom on new growth, so there is no point in keeping any of the previous year's growth. Then they grow back from the roots.

I suggest replacing it. They grow quickly and it sounds like the other one is worn out. In case it is diseased, try to plant a new one in a different spot. If you want to try to revive it, these articles should help:

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/how-to-revive-butterfly-bush.htm

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-diseases.htm

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/fertilizing-a-butterfly-bush.htm

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-care.htm

Your zone is warm enough (7b to 8a) that you should only need a 2 to 4 inch layer of mulch beneath the plant during winter. Leave the plant unpruned during winter to help protect it. Then in late winter or early spring, cut the plant down to about 8 inches from the ground. New growth will soon appear and your plant will bloom on the new growth. You can leave the mulch beneath the plant year round. These articles should help with its care.

https://hgic.clemson.edu/factsheet/butterfly-bush/

https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/overwinter-a-butterfly-bush.htm

Moving them now might not give them enough time to reestablish before winter (you would need 6 to 8 weeks). I suggest waiting till spring to move them. You only can split them if they have separate trunks.