First, a comment: you only need to worry about signal collision if the frequencies are the same. The flexNT and flexDF are both high frequency (13.56 MHz). The x-Series comes in both low frequency (125kHz) on the xEM and xBT, and high frequency on the xNT, xM1+, and xIC (double check me on the xBT and xIC, but I’m pretty sure.) If your chips are different frequencies, you don’t need to have any concerns.
Assuming both chips are using the same frequency, you could potentially have issues. With the x-Series the coupling distances are very short (I generally have to have the reader directly against the implant) while the flex implants can have a few centimeters range. Personally, in my left hand I have an xNT in the webbing between my thumb and pointer, as well as a flexDF on the back of my hand above the tendon in my pinky finger (so on the far side of the hand.) Very rarely have I had an issue, although I do use the xNT for logging in to my laptop (with the awesome Dangerous KBR1!!) and occasionally have had to scan twice because the KBR1 has slightly preferred to read the flexDF (with its better read distance) than the xNT.
Anyway tl;dr, probably not.