Have you put the two piggies on neutral territory and bonded them properly so they are now living together without bars between them?
If they still have bars between them then they are not bonding properly yet, and you will need to go through the bonding process.
You need to put them somewhere neutral (outside of the cage -in a pen on your bathroom or kitchen floor for example) for several hours for them to see if they like each other and can form a bond.
If all goes well after those few hours, then you move them together into the same cage and leave them together from then on. It then takes two weeks of being in the same cage for them to fully form their bond.
Rumble strutting, chasing, mounting, the squeaking sound (which is called submission squeaking) is all perfectly normal and something they will do throughout their entire lives. You will notice it a lot more after they are first bonded and certainly throughout the next year to 18 months while they are teenagers.
Simba is at the height of his teen hormone output and Mango hasn’t even started his teens yet (doesn’t start until 4 months of age) so you are going to see a lot of dominance behaviours.
A key part is for them to have a lot of space. Their cage for the two of them needs to be 180x60cm or a 5x2 c&c cage.
(It’s done now but Mango should not have been quarantined as he is too young to ever be alone. You also didn’t need the second cage given mango’s age - they should have been bonded on neutral territory on the day you brought mango home.
- added this for future information and for anybody else who comes across this thread)
The guides below will help further
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
A Comprehensive Guide to Guinea Pig Boars
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?
Dominance Behaviours In Guinea Pigs