The regulations give a list of 12 criteria applied in descending order. They are summarised here for brevity purposes, see the full text for details.
a. higher number of points
The teams are tied.
b. greater goal difference among matches between tied teams
c. greater goals-for among matches between tied teams
d. greater away goals-for among matches between tied teams
e. if any ties remain, repeat the above until no further separation is possible
Since there was no game between the tied teams, these criteria have no information to make a decision with. The teams remain tied.
f. greater goals-difference in all matches
This is determined solely by the scores in each team's win, since a draw does not affect goals-difference. If one team had a greater winning margin, they are placed second, otherwise the teams remain tied.
g. greater goals-for
The teams remain tied.
h. higher number of goals scored away from home in the group stage
This is a simple counting. The team who scored more goals during their away game(s) is placed second. A team with no away games cannot be placed second under this condition. A team with two away games cannot be placed third under this condition.
Otherwise, if the number of away goals is the same, the teams remain tied.
i. higher number of wins
Both teams have one win and one draw. The teams remain tied.
j. higher number of away wins
The team who won away is placed over the team who won at home, if this happened.
If both won at home or both won away, the teams remain tied.
k. lower disciplinary points
This criteria is based on the number of yellow and red cards received by a team. A red (including for second caution) is worth three points and a yellow is worth one point. The team with the fewest points is placed second.
If the teams have the same number of points (but not necessarily the same number of each type of card) they remain tied.
l. higher club coefficient
These are determined by formula listed in Annex D of the regulations. Section D.7 gives the final tiebreaker method:
which is eventually guaranteed to break the tie.