Is there any way to get MEP2015 (64bit) to open X10 rather than X9 if both are installed?

No, unfortunately there isn't. The installation of Prox10 overwrites the files of ProX9 in the plugins folder of Magix Mep or VPX.
... The two softwares communicate one another through "Camelot" library , since Xara DPX10 installs some files
(called "CamelotPlugin.vxx" and "CamelotPlugin_Xara_Designer_Pro_X10_x64.vxx") into the Magix common folder located in
C\ProgramData\MAGIX\Common\VideoImportPlugins

Could the problem be with the 64 bit version of X10 and not with the 32 bit version?

No, I have tried with both version 10 (64 or 32 bit) ... it doesn't work

Max