Reading TMRxH or TMRxL while the timer is running from an external asynchronous clock will ensure a valid read (taken care of in hardware). However, the user must keep in mind that reading the 16-bit timer in two 8-bit values itself poses certain problems, since there may be a carry-out of TMRxL to TMRxH between the reads.
For writes, it is recommended that the user simply stop the timer and write the desired values. A write contention may occur by writing to the timer registers, while the register is incrementing. This may produce an unpredictable value in the TMRxH:TMRxL register pair.