how do I connect two lipo batteries in order to increase endurane safely?

how can i connect 2 lipo batteries of 3.7V 240 mAH to increase endurance of system? I mean do i need to connect in series or parallel? WHAT WILL BE OUT PUT VOLTAGE AND CURRENT IN BOTH CASES?

Your thread title is unhelpful and gives no indication of the problem you are trying to solve. Many members here will simply ignore posts where the title suggests the poster is impatient.
The two batteries store a certain amount of energy E. That is the absolute maximum a 'system' can extract from them, no matter how you connect them. So a system which uses energy at a rate P will result in an endurance T= E/P. Pick your own units for E,P,T .

In parallel you want two identical batteries, you will get the same voltage as one battery and close to double the current (AH Rating). In series you get the sum of the voltages but the current rating of a single battery. I suggest you get acquainted with **broken link removed** to better understand batteries in general. What output current and voltage did you want? If you parallel two identical batteries once they run down they can be charged as a parallel pair in the future.

Ron

When combining cells, the degree of mismatched ESR and SoC affects reliability. For serial, voltage sensing on charger per cell is needed, which is why they have 3 pin on multi cell batt's. Serial, doubles ESR and ESR rises rapidly when any cell approaches 10% SoC so reverse protection is often used or better, cell equalization FETs.

For parallel same thing. If not matched by factory, parallel cells can age faster by current hogging and if not matched Voc, results in self discharge until equal, which can result in excessive current as ESR is almost a short circuit. In this case if cells are of unknown age and capacity, then series equalization Rs is added to match ESR of all batteries, which may be in range of 100 mOhm and now adds more loss but more reliable.

In series the weakest cell drains fastest. In parallel, strongest battery drains faster until equal, but mismatch weakens initially faster and Rs if added is more lossy. So factories may make arrays in Series parallel like LEDs 2S2P for more capacity and resilient voltage to match load.

Rule of thumb is cell ESR must be 10% of motor coil ESR for fast start and low dip.
But in cars it is often 33% so starter current in coldest winter allows 4V drop from 12V when battery ESR is highest.(cold) thus the infamous CCA lead acid test at 0deg C and7.2V