Thank youEasy, place one (the uppermost) lace base slightly on top of the other, use invisible thread with a needle and sew together by hand. I usually do two passes, the first go longer distance with the thread on the underside then come back through with the needle closer from the topside. When finished sewing brush the hair on the uppermost overlapping piece backwards against the vented direction a few times to make sure none is pinned under the thread. Then brush it back down in the normal direction so it covers the join.
I've been doing this successfully for a few years now with my full caps to double and even tripple their life span. Added new fronts, backs, sides and even added carefully shaped pieces to bulk up the density when shedding occurs on originall base. Just keep your old used units to salvage a re purpose bits from them. Dye the completed work if needed to overcome any slight colour variations.
BA showed me how to do this many moons ago, and it's been a godsend in the present pandemic situation when ordering new custom full cap all lace wigs is not possible.