Minimizing a multiple output circuit with K-maps - and without
Good job on $f_a = \overline acd \lor bc \lor bd$.
With $f_b$, I'd denote it as $b\overline d\lor b\overline cd$ - you missed overlap with $\overline d$: $f_b = b\overline c \lor b\overline d$.
I'd tally that as and inputs: 11 complemented: 3 ands: 5 gates/ors: 2  

In the combined implementation, groups are still allowed to overlap: both blue ones can be twice as large, use one input less.
One can share either $bc\overline d$:
$f_a = b\overline cd \lor \overline acd \lor bc$
$f_b = b\overline cd \lor b\overline d$
or $b\overline cd$:
$f_a = bc\overline d \lor \overline acd \lor bd$
$f_b = bc\overline d \lor b\overline c$
and inputs: 10 complemented: 3 ands: 4 gates/ors: 2