You can safely implant two HF tags (Spark & VivoKey Flex) in the same arm without interference. You would only have to worry about collision (the phone fighting over which tag it wants to communicate with) if the tags were less than an inch apart. Even then, they would likely require different orientations of the reader antenna to couple appropriately.
I have a NExT in the webbing of my hand and a flexDF on top of my forearm behind the wrist ~3" away and I never have to deal with crosstalk, even when using a powerful ACR122U reader.