i believe with irsend and multiple lirc devices attached, you have to give the lirc node in the command line. Without this it defaults to the first which may or maynot be the blaster.

The device would be something like /dev/lirc0 or /dev/lirc1 or /dev/lirc2 and so forth

since it defaults to /dev/lirc try something like
irsend -d /dev/lircd1 SEND_ONCE i Motorola_dsr_series 1

[root@mythtv-server ~]# irsend -d /dev/lircd1 SEND_ONCE i Motorola_dsr_series 1
irsend: could not connect to socket
irsend: No such file or directory

So I was thinking of something else when talking about the device node, usind -d /dev/lirc[0-9] will not work
Do you have /var/run/lirc/lirc0 or /var/run/lirc/lirc1 ?
If so try using that path for the device node

So I have done some testing and it is possible to get a mce receiver and serial blaster working together.
Granted my testing was on R8 but the remote system didn't change between r7.4 and R8.

So the main thing here is that the blaster MUST BE /dev/lirc0. If it's not /dev/lirc0 the transmitting will fail.

Try this:
1) boot system
2) sv stop remotes
3) rmmod lirc_serial
4) unplug the mce receiver
5) rm /dev/lirc*
6) sv start remotes (this will load lirc_serial), check that /dev/lirc0 is present. If it's present then proceed otherwise try modprobe lirc_serial and continue
7) make sure blasting works
7) sv stop remotes
plug in mce receiver
9) sv start remotes
10) test and hopefully it all works.

At the moment I don't have a way to do this automated fashion, udevs helpfulness is getting in the way.