you probably heard this already but no you can not do that  you will destroy the front end of the second receiver if you try that on the same frequency. you have to disconnect the antenna before you transmit.  I can't tell you about the external receiver jack but it sounds interesting
On Dec 5, 2012, at 12:53 PM, Michael Ryan <[log in to unmask]> wrote:

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> 
> Begin forwarded message:
> 
>> From: Michael Ryan <[log in to unmask]>
>> Date: 5 December, 2012 1:59:17 PM NST
>> To: Michael Ryan <[log in to unmask]>
>> Subject: Older rigs with external receiver operation option
>> 
>> Hi all: 
>> 
>> I'm wondering how the external receiver operation option worked? 
>> Ie: my FT-102 has a switch on the back labeled external receiver and my TS-940 had one as well. 
>> Was the purpose of this to use the external receiver to listen, while sharing a single antenna and when the controlling rig was keyed, the external receiver muted?  
>> Did you have to use 2 separate antennas as well in this kind of a setup? Once I get my antenna situation figured out, I'm thinking of exploring the idea of an external receiver to use with my 102 as the remote VFO option is vary rare to find. 
>> Certainly could do that anyway without physically connecting up a receiver to the 102, just turn the 2ND rig down when transmitting. (HIHIHI) 
>> Would like to be ready for the CY0IOF expedition. 
>> 
>> 73: 
>> 
>> Michael DE VO1AX 
>> 
>>