他们俩的性格差不多,挺像双胞胎一样: Can we use 挺 in this manner?
'挺像 X' means  'quite like X' 

'像 X 一样' means 'as same as X'

Either "他们俩的性格差不多, 挺像双胞胎的."  or  "他们俩的性格差不多, 就像双胞胎一样". would make better sentence

就 implies 'exactly'. It is a more suitable adverb for 一样 (the same) than  挺 (quite)

Note:

"他们俩的性格差不多" is grammatically correct , but logically, personality has nothing to do with being twin or not.