A tutorial in soloing a sistered 2×6.
Lay out the new beam. Cut to length.
Cut the receiving notch on the 2nd-floor sill:
Notch the beam ends for the support points:
Prepare a support bracket near one end:
Prop the beam on a convenient object.
Push it up into the waiting notch…
…and raise the other end onto the support bracket.
Hammer a brace up to position the beam against the upper floor.
Use a second brace to close the final gap. Use nailer to fasten, then bolt.
Final view of all three sistered beams for the stairway gap.
View from above.