Start two mongod's on "host":
1> bin/mongod --rest --replSet yo/host:27018
2> bin/mongod --dbpath /data/db2/ --port 27018 --rest --replSet yo/host

> cfg = {_id: "yo", members: [{_id: 1, host: "host"}, {_id: 2, host: "host:27018"}]}
> use admin
> db.runCommand(

{replSetInitiate: cfg}

)

Result of command is

{info: "Config now saved locally. Should come online in about a minute.", ok: 1.0}

,

but log of host says:
[conn3] Tue Jul 13 17:17:58 replSet replSetInitiate admin command received from client
[conn3] Tue Jul 13 17:17:58 replSet replSetInitiate config object parses ok, 2 members specified
[initandlisten] Tue Jul 13 17:17:58 connection accepted from 64.186.132.52:33731 #4
TEMP hb res cfg change:

{ rs: true, hasData: true, errmsg: "still initializing", ok: 0.0 }

TEMP hb res cfg change:

{ rs: true, hasData: false, errmsg: "still initializing", ok: 0.0 }

[conn3] Tue Jul 13 17:17:58 replSet replSetInitiate all members seem up
[conn3] Tue Jul 13 17:17:58 replSet info saving a newer config version to local.system.replset
[conn3] Tue Jul 13 17:17:58 replSet replSetInitiate config now saved locally. Should come online in about a minute.
[startReplSets] Tue Jul 13 17:18:01 Assertion failure lockedByMe() db/repl/rs.cpp 210
0x525613 0x52ee0f 0x6441fe 0x64481c 0x644daa 0x645e99 0x647f19 0x7dcfd0 0x2adfc821dfc7 0x2adfc8b9a64d
bin/mongod(_ZN5mongo12sayDbContextEPKc+0xb3) [0x525613]
bin/mongod(_ZN5mongo8assertedEPKcS1_j+0x10f) [0x52ee0f]
bin/mongod(_ZN5mongo11ReplSetImpl14initFromConfigERNS_13ReplSetConfigE+0x3de) [0x6441fe]
bin/mongod(_ZN5mongo11ReplSetImpl17_loadConfigFinishERSt6vectorINS_13ReplSetConfigESaIS2_EE+0x6c) [0x64481c]
bin/mongod(_ZN5mongo11ReplSetImpl10loadConfigEv+0x3da) [0x644daa]
bin/mongod(_ZN5mongo11ReplSetImplC2ESs+0x7b9) [0x645e99]
bin/mongod(_ZN5mongo13startReplSetsEv+0x1b9) [0x647f19]
bin/mongod(thread_proxy+0x80) [0x7dcfd0]
/lib/libpthread.so.0 [0x2adfc821dfc7]
/lib/libc.so.6(clone+0x6d) [0x2adfc8b9a64d]
[startReplSets] Tue Jul 13 17:18:01 replSet error loading configurations 0 assertion db/repl/rs.cpp:210
[startReplSets] Tue Jul 13 17:18:01 replSet error replication will not start
[startReplSets] Tue Jul 13 17:18:01 replSet error fatal error, stopping replication
[startReplSets] Tue Jul 13 17:18:01 replSet caught exception in startReplSets thread: assertion db/repl/rs.cpp:210

and log of host:27018 says:
[startReplSets] Tue Jul 13 17:18:06 Assertion failure lockedByMe() db/repl/rs.cpp 210
0x525613 0x52ee0f 0x6441fe 0x64481c 0x644daa 0x645e99 0x647f19 0x7dcfd0 0x2b7dfcef6fc7 0x2b7dfd87364d
bin/mongod(_ZN5mongo12sayDbContextEPKc+0xb3) [0x525613]
bin/mongod(_ZN5mongo8assertedEPKcS1_j+0x10f) [0x52ee0f]
bin/mongod(_ZN5mongo11ReplSetImpl14initFromConfigERNS_13ReplSetConfigE+0x3de) [0x6441fe]
bin/mongod(_ZN5mongo11ReplSetImpl17_loadConfigFinishERSt6vectorINS_13ReplSetConfigESaIS2_EE+0x6c) [0x64481c]
bin/mongod(_ZN5mongo11ReplSetImpl10loadConfigEv+0x3da) [0x644daa]
bin/mongod(_ZN5mongo11ReplSetImplC2ESs+0x7b9) [0x645e99]
bin/mongod(_ZN5mongo13startReplSetsEv+0x1b9) [0x647f19]
bin/mongod(thread_proxy+0x80) [0x7dcfd0]
/lib/libpthread.so.0 [0x2b7dfcef6fc7]
/lib/libc.so.6(clone+0x6d) [0x2b7dfd87364d]
[startReplSets] Tue Jul 13 17:18:06 replSet error loading configurations 0 assertion db/repl/rs.cpp:210
[startReplSets] Tue Jul 13 17:18:06 replSet error replication will not start
[startReplSets] Tue Jul 13 17:18:06 replSet error fatal error, stopping replication
[startReplSets] Tue Jul 13 17:18:06 replSet caught exception in startReplSets thread: assertion db/repl/rs.cpp:210