We are sorry to hear you are crash while opening two documents in XD. Would you mind sharing the OS and XD versions of your machine? Could you please generate Adobe logs and Crash dump along with the XD file and upload them to a shared location such as Creative Cloud or Dropbox and share the URL with us over a PM?
There is dump file base on tutorial on collecting crash dump file. Maybe for additional info, the app is never crash it just not responding, and its always end task in task manager by my self. And before 38.0.12 update i ussualy openning 2-3 XD files (sometimes 4) at the same time and my PC never have any problem with high RAM and proccessor usage.
Any way there is another XD update last week and its make better, but some times its happen again.