The Cure supported on the '79 tour and Robert Smith also played with the headline Banshees on a temporary basis. On the '82 tour The Cure didn't support but Robert was drafted in because of an 'illness' that was afflicting John McGeogh (according to the tour programme anyway).
I was at the '79 gig. The original guitarist and drummer, McKay and Morris, walked out of the band mid tour in Aberdeen. Robert Smith stepped up to cover guitar and I'm fairly sure that was the point at which Budgie was drafted in on drums. The other supprt that night was Spizzenergie.
For 82, Wikipedia has the following; "At that time, McGeoch was struggling with alcohol problems, and was hospitalized on his return to a promotional trip from Madrid. The band fired him shortly thereafter. Severin asked Robert Smith to take over guitarist duties again; Smith accepted and rejoined the group in November 1982"