The first attack happened in Wellfield Road, Benwell, at its junction with Canning Street, shortly before 8pm yesterday.
A 25-year-old man was approached from behind by a gang of three or four teenagers and attacked.
He was knocked unconscious and suffered serious head injuries.
He is currently in the city's Royal Victoria Infirmary, in what police described as "a non-life-threatening condition".
Sunderland danger driver jailed after leaving pedestrian 'with blood gushing from head' and hospitalised with collapsed lung
"He was unique and one of a kind" - Family pays tribute to 'much-loved' cyclist who died in suspected hit and run collision in Sunderland
Striking new mural by Frank Styles unveiled on landmark Sunderland building Swan Lodge
Violent Washington robber broke ex-partner's jaw and raided South Shields newsagents armed with knife
Sunderland families have their say on council's call for removal of 'unauthorised tributes' from loved ones' graves in city cemeteries
The second attack happened shortly afterwards in Ellesmere Road.
Police believe the same group assaulted a 27-year-old, leaving him with a broken jaw.
The gang were in their late teens, and is thought to have comprised two or three males and one female.
It is possible they may have been armed with a weapon of some kind.
Inquiries are ongoing into the attacks, and anyone with any information is asked to contact police on 101, quoting reference number 979 22/01/17.