One thing engineers have to keep in the back of their mind is how their measurement techniques affect the circuit under test. For instance, an analog voltmeter is usually rated at 20,000 ohms per volt for dc, maybe 5,000 ohms per volt for ac. What this means is you take the full scale reading and multiply by the rating. This gives a resistance of 200,000 ohms on the 10 volt dc scale, for example. So, if your load resistor is 100,000 ohms you have changed the circuit by adding the voltmeter. If the load is 10 ohms or a 12 volt battery, it probably doesn't add any significant loading. In a similar fashion an ammeter has some small shunt resistance. When current flows through it, a voltage drop appears across it. If this voltage is small enough we don't have to take it into consideration. If neither usage of the meter causes concern for the circuit under test then the measurements can be done using one meter with a great deal of confidence. If that is not true then using two meters has changed the circuit anyway and the measurements are wrong. In most practical situations ten per cent accuracy is good enough.