I'm not aware of any changes in this regard from previous version to v6.5 (which we are currently on). Each tab will only contain one dashlet (e.g. chart) and I've never seen multiple separate charts in one tab. If you grab one of the tabs and drag it to any corner it will snap into place and you will have both tabs/dashlets visible (you can then hide or change the headers to get a presentation you prefer)
Another thing is if you add multiple series into a single chart, you can set the split option to split by series which will break it into two charts and they will stay in that same tab/dashlet.