The 'do both' is not meant to last forever, just long enough to be able to decide whether your new idea really has as many 'legs' as you think it does or whether it is just folly.<p>You have to come to some kind of closure with stuff like this otherwise it will nag you forever.<p>So, after a couple of weeks/months of trying to shoot holes in it there is more than a small chance that the answer is 'it sucks'. If not then you have to decide which one of your 'children' you want to keep.<p>At that junction you have several options:<p><pre><code>  - sell your current project and use the proceeds to 
    fund the next 

  - spin off the other one to a bunch of people that 
    you trust

  - kill it anyway, because your 'old' project has in 
    the meantime gotten significantly more traction.
</code></pre>
You basically buy yourself time and and gain more knowledge to make a much more informed decision.