I never leave mine unattended. I hold the female and assist the male if needed. Some may need guiding, some may need a boost, some are just fine and are able to do their job with me just holding my female.
If they tie, even if in a pen..the female can hurt the male or even vise versa, so its best to always assist, plus then you know for sure that there is a tie and you can then calculate an expected whelp date too
Will you try the pair again? Well wishes to their next breeding!