How to use 2 N310 for TX and RX

[Johannes Demel]

Hi all,

I have a flowgraph where I want to use two N310s for TX and RX.

If I run `benchmark_rate`, everything works fine.

```
./benchmark_rate --pps external --ref external --rx_channels "0,4" 
--tx_channels "2,6" --rx_rate 61.44e6 --tx_rate 61.44e6 
--args="addr0=192.168.21.218,addr1=192.168.20.213,master_clock_rate=122.88e6
```

It's important that I use one RX and one TX channel each on those USRPs.

But it seems like I can't do that with

```
[INFO] [UHD] linux; GNU C++ version 9.3.0; Boost_107100; 
UHD_3.15.0.0-62-g7a3f1516
[INFO] [MPMD] Initializing 2 device(s) in parallel with args: 
mgmt_addr0=192.168.20.213,type0=n3xx,product0=n310,serial0=319841B,claimed0=False,mgmt_addr1=192.168.21.218,type1=n3xx,product1=n310,serial1=3180AF3,claimed1=False,addr0=192.168.20.213,addr1=192.168.21.218,master_clock_rate=122.88e6,clock_source=external,time_source=external
[INFO] [MPM.PeriphManager] init() called with device args 
`time_source=external,clock_source=external,master_clock_rate=122.88e6,product=n310,mgmt_addr=192.168.20.213'.
[INFO] [0/Replay_0] Initializing block control (NOC ID: 0x4E91A00000000004)
[INFO] [MPM.PeriphManager] init() called with device args 
`time_source=external,product=n310,master_clock_rate=122.88e6,clock_source=external,mgmt_addr=192.168.21.218'.

[...]

[INFO] [MULTI_USRP]     1) catch time transition at pps edge
[INFO] [MULTI_USRP]     2) set times next pps (synchronously)
[INFO] [MPMD] Initializing 1 device(s) in parallel with args: 
mgmt_addr=192.168.21.218,type=n3xx,product=n310,serial=3180AF3,claimed=True,addr=192.168.21.218,master_clock_rate=122.88e6,clock_source=external,time_source=external
[ERROR] [RPC] Someone tried to claim this device again (From: 192.168.21.34)
[WARNING] [MPM.RPCServer] Someone tried to claim this device again 
(From: 192.168.21.34)
Traceback (most recent call last):
  File "gr-tacmac/examples/usrp_multi_test.py", line 360, in <module>
    main()
  File "gr-tacmac/examples/usrp_multi_test.py", line 338, in main
    tb = top_block_cls()
  File "gr-tacmac/examples/usrp_multi_test.py", line 133, in __init__
    self.uhd_usrp_sink_0 = uhd.usrp_sink(
RuntimeError: RuntimeError: Error during RPC call to `claim'. Error 
message: Someone tried to claim this device again (From: 192.168.21.34)
```

I can share the example flowgraph. It's just a USRP source connected to 
a Qt time sink (and a Qt Freq sink). And a USRP sink that's fed by two 
Signal sources.

It works if I only have either a USRP sink or USRP source. But in case I 
try to use both. The configuration breaks with the above error. How do I 
fix this?

Cheers
Johannes