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Receive on both channels

One command records RX1 and RX2 together, sample-aligned, in one file.

# run from: the repo root
tools/sigmf-capture.py record out --channels both --rate 3e6 --seconds 5 --verify

One file, core:num_channels: 2, interleaved RX1-I, RX1-Q, RX2-I, RX2-Q per sample instant. --split writes two single-channel recordings instead.

Good to know
rate 2 channels at 3 MSPS are clean; at 10 MSPS they drop samples
with the FPGA ÷8 decimator both channels are filtered (patch 0021, applied by every build): at least 69.7 dB of alias suppression on channel 1
IIO channel names cf-ad9361-lpc input voltage0/1 = RX1 I/Q, voltage2/3 = RX2 I/Q; gain, rate and bandwidth live on ad9361-phy

Two panels measured on the board. Left: the swept anti-alias response of channel 1 from 0.2 to 20 MHz. The stock build is flat at plus 1.4 dB across the whole sweep — no attenuation at all. The patched build sits at minus 4.6 dB through the passband, rolls off at the 3.84 MHz window edge, and reaches about minus 70 dB beyond 5 MHz. Right: the spectrum channel 1 delivers for a 10 MHz tone at 7.68 MSPS. The stock build has a tall spike at plus 2.32 MHz — the alias — while the patched build shows only a small DC bump and noise. Two panels measured on the board. Left: the swept anti-alias response of channel 1 from 0.2 to 20 MHz. The stock build is flat at plus 1.4 dB across the whole sweep — no attenuation at all. The patched build sits at minus 4.6 dB through the passband, rolls off at the 3.84 MHz window edge, and reaches about minus 70 dB beyond 5 MHz. Right: the spectrum channel 1 delivers for a 10 MHz tone at 7.68 MSPS. The stock build has a tall spike at plus 2.32 MHz — the alias — while the patched build shows only a small DC bump and noise.

Channel 1 with the decimator engaged, before and after patch 0021.

Coherent, but not calibrated

Both receivers share one RX_LO, so their phase relationship is stable. The analogue phase offset through baluns and traces is real, tens of degrees and frequency-dependent: measure it with a splitter and matched cables before trusting any angle.

Without the capture tool? 
# run from: your host
iio_attr -u ip:192.168.2.1 -i -c cf-ad9361-lpc voltage0 sampling_frequency 7680000

# channel 0 is voltage0/voltage1, channel 1 is voltage2/voltage3
iio_readdev -u ip:192.168.2.1 -b 131072 -s 262144 cf-ad9361-lpc \
  voltage0 voltage1 voltage2 voltage3 > both.bin

More: capturing IQ · two receivers that both survive decimation.