I took both french and german gcse, and although I got a 9 in french and only a 7 in german, I didn't find the workload too much at all. It's all down to personal preference. I took the two of them cos out of all the alternatives they were the ones I enjoyed the most so I didn't hesitate to pick them. If you enjoy both french and spanish equally, and prefer them over the alternatives, then there's nothing wrong with taking both. At gcse, it's all about studying what you enjoy.
they're definitely great subjects to take if you are interested in them