Installing and enabling both master and slave semisynchronous plugins on same server
This will not cause any problems. I have always setup all MySQL 5.5. DB servers for clients with both plugins. In fact, I answered a question back on Aug 05, 2011 where I posted an update on Aug 08, 2011 on how to install both plugins at the same time
Please see Is MySQL Replication Affected by a High-Latency Interconnect?
Here is the excerpt from that update
EXCERPT
The configuration of MySQL 5.5 Semisynchronous Replication is straightforward
Step 1) Add these four(4) lines to /etc/my.cnf
[mysqld]
plugin-dir=/usr/lib64/mysql/plugin
#rpl_semi_sync_master_enabled
#rpl_semi_sync_master_timeout=5000
#rpl_semi_sync_slave_enabled

Step 2) Restart MySQL
service mysql restart

Step 3) Run these commands in the MySQL client
INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
INSTALL PLUGIN rpl_semi_sync_slave  SONAME 'semisync_slave.so';

Step 4) Uncomment the three rpm_semi_sync options after the plugin-dir option
[mysqld]
plugin-dir=/usr/lib64/mysql/plugin
rpl_semi_sync_master_enabled
rpl_semi_sync_master_timeout=5000
rpl_semi_sync_slave_enabled

Step 5) Restart MySQL
service mysql restart

All Done !!! Now just setup MySQL Replication as usual.