It would be nice if Portfolio could track that the working hours of a person do not exceed a certain value even if the person is member of more than one team.

One possibility would be to manage the general availability of a team-member not with presence and absence but instead manage it exactly like the team availability. Thus with a weekly hours field and exceptions. This would then allow to divide the working hours of a person between two teams automatically. If nothing is specialized in the team availability section this could be dividing the hours between the teams. If in the team availability section hours or a percentage is specified this value would be chosen and for the other teams the remaining hours are used.