As I see it there is a simple choice, either try to study two languages at once or accept that once learned a language will be forgotten if not used and will need to be revived at intervals.
My experience so far this year supports the second option: I went to Caen in June for the D-Day commemoration weekend and ended up helping a Chateau tour guide (who didn’t speak English) as a "better than nothing" translator (French into English) for a couple of hours. Immediately after this I met a trilingual friend and I couldn’t think of a thing to say to her in Spanish, even though I had emailed her in Spanish three days earlier.
The lesson: learning two at once doesn’t work for me, it might if I was surrounded by trilingual friends and lived in a trilingual country but I don’t. So the strategy I propose is to take short "holidays" from Spanish during which time I revive French, typically for a week or less before visiting France or a country where French is spoken. On return to England I go straight back to Spanish. I am not sure how this will work long term but for now I hope that French can take the hit of being put in second place for a couple of years.