Is it possible to select multiple markers on a single map (Google or OSM)?

Also, I was wondering if it is possible to create a custom question type to do something like this? I found some documentation here: manual.limesurvey.org/Creating_a_question_plugin but it does not look to be complete or have examples. Anyone have experience with this? Is there a way I can add another column to a database table so that I can store two different locations? Thanks.

This isn't possible with the core map type question. A possible solution would be to use a multiple-short-text question and create your own Google or OSM map. The challenge you will face will be what to do if the respondent clicks a third time or wants to move a marker. I think it would be far simpler to use two map questions.