Neither are 'idling bastards'. They are both good workers which makes it worse. I have been training one of our receptionists to do some of the data entry work. Not difficult work once you get into a routine but time consuming, that should help.
Regarding the employee off on maternity leave there is very little you can do to solve that situation at the moment apart from employing some extra help.
The colleague with personal problems - if he is a valued worker then you want him back, but you will have to show some understanding in this situation. I know you would have gone into work and I have also done the same in similar circumstances - I would see it as a distraction and something else to focus my mind on. However some people cannot function this way. Would it be even more frustrating for you if he was sitting in the office unable to do a thing and becoming emotional etc.
Are you in contact with him at the moment and is it appropriate for you to discuss this with him?
Perhaps suggesting a return date for him to have something to aim towards , stressing he is a valued member of the team - I know this is quite touchy feely - but he may need to feel he has some kind of use or is there anything he can work on at home etc
If this is a rare blip in his work history then persevere and help him. He should hopefully always remember the thoughtfulness and consideration shown to him at this time.
We're all different but we've all been there