It depends a little bit on what is meant by safe. Technically speaking, the surgery is just the same whether a multi- or mono-focal lens is used, with the same very low risk of complications.
The question rather, is - can you get used to the different visual experience between the 2 eyes?
So the original eye with a mono-focal lens will see very clearly at one distance. At other distances it is somewhat blurry unless you wear the necessary glasses.
If you get a multi-focal lens in the other eye, you will notice differences between the eyes, some of which are more obvious than others.
1. The eye with a (diffractive) multi-focal lens will see rings around light sources at night (or when the background is dark).
2. Without glasses, things are clearer at different distances with the multifocal lens.
3. With glasses, things could feel a bit sharper at the focal distance of the monofocal lens, because the monofocal lens is always going to give better contrast at its focal distance compared with the multifocal lens.
Generally speaking, having a monofocal in one eye and a multifocal in the other eye is not encouraged, because many people are likely to find it difficult to get used to the different feelings between the 2 eyes. If they cannot get used to it, a second surgery to replace the multi-focal with a mono-focal lens would be necessary.
