Welcome to the forum! Look at those beautiful boys of yours!!! Happy to hear you want them to enjoy the outdoors safely! I think you can begin putting the jackets on them and let them wear them inside the house -- as soon as the jackets arrive! They need to be completely comfortable wearing them before you attach a leash and head out the door. Hopefully, you will have someone walking one of the cats as having two of them going in opposite directions is not going to work. Know that initially, your kitties will stick close to the house. They will investigate and sniff everything. They will stop often. It's not really a "walk."
. BUT .... as they gain confidence, they will want to walk farther and farther. My bengal and I started off just in our yard. Now we walk about 6 to 8 blocks. Carry a large towel with you in case you all want to sit and rest.
Just know that once you start the walks -- they will insist on them and want to go back outside constantly. Thus, you might want to see about getting a catio or cat enclosure that they can enjoy on their own terms. If you cannot do a permanent enclosure, there are portable enclosures that fold up and store away when not in use. It just gives you a little freedom -- not that you should ever leave the cats in a portable enclosure without your being right there. And you never tie your leashes up outside and leave the cats to "run."