In short, no. Sources cannot be combined with each other. It's only possible to create galleries with one source at a time. The primary reason is that the date information is not standardized across sources, therefore interleaving distinct entries by date would likely be problematic. The future may hold the possibility of merging multiple sources. However, the following ways are available already:
- Combine multiple RSS feeds together. It will automatically sort the entries by date. For example, a YouTube and a Vimeo channel's combined video gallery. Create a combined portfolio from your most recent 500px content and the latest deviations from DeviantArt.
- Different sources can be used in subsequent instances on the same page. There is nothing stopping you from placing multiple JIG galleries under each other. They'll appear to seamlessly merge where they touch, especially if you use the JIG settings > General tab > Incomplete last row setting's Hide option - for a gapless look.