As long as both of them know and nobody is getting hurt in the process it should be ok. Most of my friends and I would not be ok with being in an open relationship.
There is nothing wrong with dating multiple people at the same time – as long as you and your partners have an open and honest communication about it.
If both partners are okay with this decision, and no feelings are being hurt in the process, a healthy relationship can be maintained. In addition, it is very important to stay healthy sexually if sexual activity is happening. Make sure to always use a barrier method (internal or external condom and dental dams) and if sex toys are being used either keep separate ones for each partner or also use a condom. STI’s can still be spread if a sex toy is not being properly clean/used.