So you currently have a camera with a built in flash that you do not use and one flashgun that is manually fired by an assistant. Presumably you set the camera's shutter to be open for a couple of seconds to synchronise the flash with the camera. The downsides of this are (1) you can get a lot of missed shots where the flash is either fired too early, too late or not at all. (2) by leaving the shutter open for a long period the camera also picks up any other light (such as helmet lights - your own or anyone else's which can cause unpleasant blurring or light trails.
So if you want to improve your setup, you have the following options:
(1) Buy a single slave unit (eg firefly) and attach it to your existing off camera flash. Cover your camera's built in flash with a bit of exposed film (supplied with the firefly last time I bought one) and use the built in flash to trigger the off camera flash whilst not revealing any visible light from the built in flash. The improvement is that you are no longer reliant on your human slave to fire the flash and can use more conventional shutter speeds. Light trails are not a problem.
(2) Buy no slave units and instead put your money towards a second flash gun. You will need a second human slave to fire it and the problems of co-ordinating the firing of both flash guns and the camera will be at least as difficult as with one flash gun. Advantages are that you have more light and a lot more options for lighting - eg combining front lighting with backlighting. The slave only serves to assist in synchonisation. You do not need a firefly in order to use multiple flashes.
(3) Buy one slave unit and one flash gun and attach them together. Continue to trigger your existing flash manually. The benefit of this is you can dispense with a second human slave.
(4) Buy two slave units and one flash gun. As per option 1 but now you have two separate off camera flashes and no problems with synchronising them.
If you do end up buying fireflies, don't forget to consider if you need the firefly 3 model or the 2. If your triggering flash has pre-flashes that cannot be turned off then you need a firefly 3. Of course if you have a firefly 3 and don't need the programming function then it has the same functionality as a 2 anyway so your bases are covered.
Ultimately you will get more from having multiple flashes. The fireflies simply eliminate a lot of the hassle. To give a simple example of variations with two flash guns, the following two photos were taken from exactly the same location:
In this photo both flashes are aimed away from the camera with the light angled from the right hand side. The second flash behind the caver lets me illuminate multiple sets of formations.
In the second photo the flash behind the caver has been turned around to point towards the camera. This backlights the steam from the caver's body and helps highlight the straws and the shape of the rock.