From a string player:
You can do it the way AntiA suggested. Though it could (unlikely) produce a moment of "why'd he do that?" because for string instruments ties and slurs almost always are a direction for bowings, though they can indicate phrasing. However, they'll get over it quickly.
Another way of doing it is to just have the wholenote and put for instance a downbow mark at the begining, and in the middle of the measure an upbow marking. This is common, especially when there is a very long held note that is tied (an example of when slurring is more phrasing, than bowing.) And actually, if you do what AntiA suggested secondly, mark the following measure, most of the players would mark thebowing this way themselves.
Another solution, if the point it two have the note following the long note to start on a particular bow, is to adjust a measure before the wholenote (using a slur or some other marking) so that going into the long note, no change is needed. Assuming here that this isn't a slow piece and a bow change isn't needed.
Though I assume the reason you want the wholenote is to avoid the second attack from two separate halfnotes and produce one continuous note. It is kind of hard to know for sure without knowing the context in which this occurs, and your reason for wanting a bow change in the first place.