Multiple GPUs perform slower than single GPU to train a semantic segmentation network

I have to my disposal two NVIDIA Tesla V100-16Gb GPUs to train a deep neural network model for semantic segmentation. I am training the Inception-ResNet-v2 network with the DeepLab v3+ architecture. I am using the randomPatchExtractionDatastore to feed the network with training data. When I set the 'ExecutionEnvironment' option to multi-gpu the processing time for each iteration is higher than using only gpu, that is a single GPU. I am working in Windows 10 with MatLab 2019b. What should I do to use the full potential of both GPUs for training? Bellow an example of my code     pathSize = 512; imageSize = [pathSize pathSize 3]; numClasses = 6 lgraph = deeplabv3plusLayers(imageSize, numClasses, 'inceptionresnetv2','DownsamplingFactor',16); MaxEpochs=10; PatchesPerImage=1500; MiniBatchSize=20; options = trainingOptions('sgdm', ... 'ExecutionEnvironment','gpu',... 'LearnRateSchedule','piecewise',... 'LearnRateDropPeriod',3,... 'LearnRateDropFactor',0.2,... 'Momentum',0.9, ... 'InitialLearnRate',0.03, ... 'L2Regularization',0.001, ... 'MaxEpochs',MaxEpochs, ... 'MiniBatchSize',MiniBatchSize, ... 'Shuffle','every-epoch', ... 'CheckpointPath', tempdir, ... 'VerboseFrequency',2,... 'Plots','training-progress',... 'ValidationPatience', 4); imageAugmenter = imageDataAugmenter( ... 'RandRotation',[-20,20], ... 'RandXTranslation',[-10 10], ... 'RandYTranslation',[-10 10]); % Random patch extraction datastore PatchSize=[pathSize pathSize]; dsTrain = randomPatchExtractionDatastore(imds,pxds,PatchSize,'PatchesPerImage',PatchesPerImage,'DataAugmentation',imageAugmenter); [net, ~] = trainNetwork(dsTrain,lgraph,options);