Many halachic decisors permit cooking meat and fish dishes simultaneously provided at least one of them is covered (SA YD 108:1). After the fact, it is permitted to eat the food even if not covered (Rema on SA YD 116:2).
When meat and fish are cooked together in a closed area, the steam
produced by one is absorbed by the other. Therefore, one should avoid
cooking meat and fish together in an oven or closed grill, unless the
fish or the meat is covered, such as with aluminum foil and the like,
so that it is shielded from the steam produced by the other.
Nevertheless, after the fact, if one did cook fish and meat together
in an oven without covering one of them, they are permissible for
dinonline brings the following
Although the Gemara (Pesachim 76b) does imply that it would a problem,
even bedieved, to cook meat and fish together in the same oven, the
fact that this halachah is not ruled by Rif or Rambam has led poskim
to be lenient. Although we continue to be careful not to eat fish and
meat together, many authorities are lenient concerning fish or meat
that absorbed the taste (alone) of the other.
For lenient opinions, see: Issur Ve’heter 39:25, as cited in Rema,
Yoreh De’ah 116; Shach, Yoreh De’ah 116:1; Taz, YD 95:3; Darchei
Teshuvah 116:16 (quoting from several authorities); Ha’elef Lecha
Shlomo 312. For more stringent opinions, see Peri Chadash (116:4);
Peri Toar (116:3); Chochmas Adam (68:1).
Lechatchilah, one should not cook meat and fish [uncovered] in the same oven, but
bedieved, one may rely on the lenient opinions.
R Yitzhak Yaacov Fuchs writes (in his book Hakashrut, p. 418)
Lchatchilah, it is forbidden to put meat and fish together in the same baking compartment in the oven even though each one is in a separate baking utensil. It would be permissible if one of the baking utensils was
covered entirely in a complete manner (wrapped all around and
hermetically closed). Bdieved [after the fact] if not fat or juices
dropped from one utensil to the other it is permissible to eat both
foods (Aruch Hashulchan YD 116:10)
R Ari Enkin summarizes
This would logically extend to restrict baking dishes with these two
types of foods in the oven at the same time, unless both pots are well
covered, lest any tastes, via the steam, spread from one to the other.
See also here at length from Eretz Hemda which concludes
while it is halachically safer to not roast meat and fish uncovered in
the same oven, it seems reasonable to do so in a regular, large oven
when there is a need