I have no idea why they do that, but that's very strange...
It is somewhat more uncommon but not unheard of that guinea pigs will drink another piggy's urine; they will sniff and lick it as the pheromones in it give them vital information about the status of the other guinea pig. As there seems to be an unusually strong latent dominance/territory component in your boars' relationship due to their initial meeting, this is likely just a stronger version of the more normal behaviour.
Please always give piggies time to settle and to get to know each other before introducing on neutral ground in order to minimise the risk of fear-aggressive overreactions during bonding; ideally that is several days of living alongside. Any guinea pig that is just put in the same cage is perceived/will feel as an intruder and the territorial aggression potential is very high (this goes for any gender) - not because the piggies are aggressive by nature but because you have put them into a dangerous situation they would absolutely avoid if they had any choice.
More about bonding and fear-aggression in this illustrated guide here:
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
I would strongly recommend that you keep an eye out on your boys as there is an underlying conflict/dominance potential, which currently only expresses itself in overactive scent marking/scenting. It can flare up anytime your dominant boy is ill/older/there are changes to the territory, i.e. whenever the hierarchy is in question or needs to be re-established.
Unlimited hay should make over 80% of the daily food intake. The more you encourage your boys to eat hay, the healthier they will be and the longer they can live. A hay based diet with a limited but balanced amount of veg and 1 tablespoon of pellets per piggy per day more as daily treats than as the mainstay of the diet can add 1-2 years to a piggy's life, keep the teeth strong, boost gut health and through this the immune system and help minimise the risk of avoidable illnesses.
Food bowls, water bottles and hideys with only one exit can also become quickly flashpoints of fights, so please have anything in twos, spaced well apart from the other same item. Feed pellets and veg only in portions that can be eaten in one go and remove the bowls in between meals.
You can find more 'do/don't' tips to help stabilise your boar bonds in this guide here:
A Comprehensive Guide to Guinea Pig Boars
Our comprehensive diet guide, which looks at all food groups:
Long Term Balanced General And Special Needs Guinea Pig Diets