Do not use basswood for a spar or anykind of a spar doubler. If the original type was not plywood (pretty common) then Spruce is a preferred type of spar material. Most model airplane spars are not plywood however any kind spar doubler or joiner the preferred material would be Birch plywood that is multi layer and often sold in shops. A common ply used in many airplanes especially arfs is called LitePly and it has a minimum layer count usually not more than three ply whereas Birch Ply in the same sizes are up to seven ply. Lite Ply as well as basswood is a really bad idea for spars or doublers as it is very, very weak.

John