You can run simultaneous Convert experiments on the same page or set of pages.
General Collision Prevention
For setting this up, first, you have to understand the order in which experiments are executed. Experiments conditions get tested sequentially on a page, taking into account their experiment ID.
Set Traffic Distribution to 50% for the first experiment
So, to achieve this you need to set the experiment with the lowest ID number, to only use 50% of the traffic. You can do this in the Stats & Settings section of the Experience Summary.
Set Audience Condition of "Bucketed in Experience is No"
Then in the second experiment, set an audience condition of "Bucketed in Experience is No". You can find this if you add a new Audience (under Visitor Data). This condition means that the visitor will be tested only if they have not been tested before. This will prevent from testing twice the same visitors.
Exclude Visitors of other Experiments
If you have more tests running on your site, you can set the second experiment to specifically prevent the tested visitors of the other test to be bucketed on the second, instead of the general collision prevention, mentioned above.
The following article describes how to do that: