Can I have multiple research jobs totalling > 1.0 FTE in the United Kingdom?
Generally in UK academic institutions, you are paid based on your "grade" on the pay scale (for posts up to but not including professor). If you were asking if you would receive more than 1.0 FTE in pay, I believe the answer is no, as you would no longer be on the regular scale. Your pay is based on your post, and the research funding would contribute to the University's costs and overheads, as well as your pay. The HR office would not let you be paid more than you should receive according to their calculations.

If you mean holding more than one Research Associate post, while still receiving the same pay, this depends on your institution. I have many colleagues working on multiple projects simultaneously. They're putting in more than 1.0 FTE of effort. As to their official FTE, I believe thy are officially 1.0 FTE, and they are a Research Associate "for" a given member of staff. Their time is then "unofficially" (for want of a better term) split between multiple projects. 

I am sure this will vary between funding sources and institutions, as to what is tolerated/the norm. In this case I reference, however, this is not a case of holding multiple RA posts; rather holding a single post, which works on multiple projects. Since their post is not tied strictly to a single project, the FTE is perhaps not the best metric to gauge them by.