Given We reserve the right to reassign seats at any time, for operational, safety or security reasons. there is no cast iron guarantee so I am interpreting your question with a little licence, ie more good probability than certainty.
I think there is a high chance of persuading someone in a Standard seat to swap with someone who has a Priority with extra legroom seat, so one of you might pay £15 and be able to persuade whoever is sitting next to the other to relinquish their seat in exchange. To increase your chances, buy a Standard seat (+£8) for the other and choose a middle seat for them. Otherwise the Standard seat may well be next to a couple who will no more want to be separated than the two of you want to be together. Of course more expensive and less certain to work than plumping for £8 x2, but that might not be an option, depending upon when the seats are chosen.
Lucian has performed an amazing amount of analysis and blogged about it in an article titled How to Get a Free Seat on Ryanair. He suggests:
First of all, in order to have a chance, you have to do the check in (almost) at the same time, otherwise there is no chance that you seat together! You can do it by using 2 different browsers/computers and check in simultaneously. If you can do it, there are 2 ways of making sure you get seated together:
(recommended) – do the check in exactly when the check in period starts! That is exactly 7 days before the flight date, at midnight (UK time). In order to fasten the check in process, you can fill the passenger details at any time before this moment. This way, you will most probably be allocated the seats 18A and 18B, over the wings (if they are available). So you will stay together.
(only if you really hate row 18!) – you first have to understand the “allocation algorithm”, as described above. Identify the current row and check whether there are any empty seats on the already allocated rows.
- If there are, you will (probably) be allocated those seats, so you will NOT be seated together. So you have to wait till these empty seats get filled.
- If there are no empty seats on the already allocated rows AND there are 2 seats available on the current row, you will be allocated those 2, go for it now!
Keep in mind that as the plane gets filled, the chance of having empty seats on the already allocated rows grows, so you may lose a lot of time keeping an eye on it. So if you can still do the first method, I strongly advise you to do it.
If neither of the above (pay or play the system) is chosen, or was chosen but did not work, there is still time for Plan B, though this is probably less likely to achieve the desired result. A low load factor should increase your chances greatly but that is unlikely, so your fellow passengers may not be in the best of moods for being herded like sheep and jammed into seats with low pitch. Plan B is of course as suggested by @Quora Feans, coupled with some incentive such as mentioned in @Dorothy's Comment, that might help overcome any grumpiness. IMO more effective than a blatant 'bribe' would be to establish some rapport before asking, such as with a joke (charm them), but I'll leave that to your ingenuity.
You would be advised to be careful with your selection however. Once one candidate has refused others may be more likely to follow precedent. So seek out someone who will at least understand why you are offering them sweets (ie is likely to speak your language – your friend might test them out in the language first, maybe "I'm not sure, is this 15B?") and, if you can, pick an aisle or window seat passenger next to one of your two allocated seats (to avoid trying to split others travelling together).