there are many ways to make one curve tangent to another in a Sketch:
1. with the Profile tool, use the Tangent Arc option
2. while drawing the second arc, use SmartPicks (blue) to impose the tangency
3. just draw the two arcs, and then constrain the second one to be tangent to the first
4. if roundng-off two adjacent corners; add the corner fillets, constrain the second radius to equal the first, constrain the first radius to equal the length/2