Jet-ski officials were reluctant to talk about the incident, which occurred about 4.30pm some distance offshore.
One brother was taken to Patong Hospital with undisclosed injuries.
The crash comes in the Seven Days of Danger, a safety campaign which usually covers Phuket's roads but this year has been extended to include marine safety as well.
A Russian tourist was killed earlier this year when a jet-ski fell back and struck him as he tried to crash through a wave at Patong beach.
Jet-skis are a controversial topic on Phuket at present, with many people saying they should be banned, as they are in the neighboring ecofriendly provinces of Krabi and Phang Nga.
Attempts to keep the scores of jet-skis by zoning Patong for swimmers and jet-skis have so far failed to isolate the dangerous jet-skis from the swimmers.
Scams continue, with jet-ski operators usually making extortionate claims whenever jet-skis are damaged.
One problem is that the insurance is not comprehensive so jet-ski operators usually attempt to make the rider pay an additional sum.
Almost always, this includes a ''time out of the water'' fee of between 15,000 baht and 17,000 baht, calculated at 3000 baht a day for the time the jet-ski could be unavailable and being repaired, plus a cost for ''paperwork.''
Quite often, the jet-ski is back in the water the following day.
Jet-ski operators have also been known to go to hospital to demand extra damage costs while riders are being treated for their injuries.
It's not known in this case how extensive the damage was to the two jet-skis but a jet-ski spokesperson said both machines were insured.
Injuries to individuals are covered up to 50,000 baht.
Phuket's honorary consuls have recommended that jet-ski operators pay higher levels of insurance to cover all repairs so there are never any arguments about additional payments or ''time out of the water'' fees.
An attempt was made to phase out jet-skis from Phuket over a period of seven years but successive governors failed to achieve phase-out targets and the machines are now in greater numbers at more beaches than ever.