To fuse two COE 90 glass pieces, both must be evenly hot.
Heating just the bottom of the bowl and top of the base would create a great deal of stress between the hot area and the cold area. The piece would probably break before it was placed in the annealer.
Have you watched furnace workers joining two pieces? One piece is "garaged" in a warm kiln while they work on a second piece (i.e. joining a cup and foot of a goblet). That is how they avoid shocking the glass. (I attached a description of a garage used for furnace work from Henry Halem's Glass Notes; however, many glassblowers use a kiln for this purpose.) This would be difficult with fused glass, especially if your pieces are large and thick.
Flameworkers using borosilicate glass have a little more leeway because the COE is lower, but they often use the "garage" technique when joining two pieces.
Consider joining your pieces with adhesive. Here is a link to our "coldworking" guide: https://libguides.cmog.org/cold
Adhesives are often used to attach fused pieces. Milon Townsend wrote several articles about different types of adhesives (listed in our guide).
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