<input type="hidden" id="gwProxy"><!Session data></input><input type="hidden" id="jsProxy" onclick="jsCall();" /><span style="font-size: 12pt;">Can I use a y harness to connect two nihm batters in parallel to get twice the mAh</span><input type="hidden" id="gwProxy" /><input type="hidden" id="jsProxy" onclick="jsCall();" /><div id="refHTML"></div>

Yes, Yes you can.

Only if they contain the same number of (series) cells.

I hate it when someone makes an unecessary qualification.

<pre>Im going to be using two Hobbico Hydrimax Ultra NiMH 6V 2000mAh in my rc boat can I use the one charging jack on the switch to charge both batt in the boat at the same time </pre><input type="hidden" id="gwProxy" /><input type="hidden" id="jsProxy" onclick="jsCall();" /><input type="hidden" id="gwProxy"><!Session data></input><input type="hidden" id="jsProxy" onclick="jsCall();" /><div id="refHTML"></div>

I'm not familiar with the way it's wired, but if it simply charging packs in parallel then usually the answer is no. The two packs' peak will not line up. I have heard, however of some success using a c/10 rate or less (the users think the packs don't "peak")

If one pack got full sooner than the other,its voltage will drop and will drain all the charge current.
So the second pack will remain partially charged and the first will get warm to lose the extra charge .
it is harmful specially at high charge current.
but when you put the packs in series, current will charge both finally.