Those are good questions:
The least expensive, usually from China, hand-held radios ("HT"), use what is known as SOC technology: System on a Chip - The whole radio is virtually built into one chip.
Most of the more expensive radios (usually 3X the cost and more), have additional circuitry known as a superheterodyne circuit. This gives the receiver both more selectivity (the ability to differentiate between two powerful signals on two close frequencies) and sensitivity (the ability to pull in more distant and/or weaker signals).
In simplex mode: one HT is transmitting on a frequency and the other HT is receiving on the same frequency. There is no second transmission to overwhelm the receiver of the second HT; only the one signal.
In repeater mode, the first HT is transmitting on 467.abcd up to the repeater; and the second HT is receiving the retransmitted reception signal from the repeater on 462.abcd.
The second HT must differentiate between two received signals: the signal you want to receive on 462.abcd from the repeater many miles away, and the HTs more powerful signal on 467.abcd. Hence the HTs signal washes out the reception of the repeater's signal on the second HT.*
Superhetrodynes have that ability; SOC's do not.
How much is far enough away? Differing factors are at play here: The power of the HT transmitter; the distance between the HTs and the distance and power from the repeater.
Experimentation will help you determine the answer.
*NOTE: this is why when two inexpensive HTs are bundled together with the appropriate cabling and settings to act like an inexpensive semi-portable repeater, they must be on the two separate Ham bands: 2 meters (144 MHz) and 70 cm (440 MHz) to avoid this washout effect.
