Can I use my iPhone's 18W adapter on my MacBook Pro? And vice versa?
Yes, it is safe to connect the 18W USB-C power adapter to a MacBook Pro. However, I wouldn't expect it to actually be useful for charging the laptop. The charger supports 5V (max 15W) and 9V (max 18W), but with the MacBook Pro you really want a 15V or 20V charger to be able to get some energy transferred in a timely manner.

And yes, it is also safe to use the MacBook Pro's USB-C charger with the iPhone 11 Pro using the correct cable. It should be able to charge your phone just fine.